To quickly switch a selector valve for a booster pump from a discharging position to a supplying position, and to miniaturize the selector valve.

An input chamber 9 of the booster pump 2 is alternately connected to a pressure port P and a return port R by the spool-type selector valve 20. The selector valve 20 is composed of a spool 25 having a large first piston 31 and a small second piston 32, a first operation chamber 41 opposite to one end of the first piston 31 and pressing the spool 25 to the other end direction, and a second operation chamber 42 formed between the first piston 31 and the second piston 32, and pressing the spool 25 to one end direction. The second operation chamber 42 is connected to the pressure port P, the other end of the second piston 32 is communicated with the return port R, and the first operation chamber 41 is alternately connected to the pressure port P and the return port R by a pilot means 60.